WD-40
WD-40 is a widely used lubricating oil that is perfect for this job. Spray it right onto the hinge, then open and close the door a few times and it'll start to do the trick almost immediately. Use the straw provided to stop the oil going everywhere (as it can stain).
If you don't have any WD-40 handy, you can try a little motor or even cooking oil, although the effects of these won't last as long.
Wire Wool
If WD-40 doesn't work and your hinge is still squeaky:
- Remove the hinge completely from the door and slide out the hinge pin.
- Give it a good rub with wire wool to remove any dirt or rust which might be causing the squeak.
- Replace the hinge on the door and re-spray with WD-40 or oil.
Soap
Another very simple solution is to take the hinge pin out and rub it with a bar of soap. Replace the hinge then open and close the door a couple of times to allow the soap to spread through the mechanism.
Replace the hinge
If a stubborn squeak just won't be silenced then it's time to chuck the old hinge out and replace it with a new one.